In some cellular mobile communication systems, such as CDMA systems (WCDMA and CDMA2000), a mobile terminal can communicate via several base stations or cells simultaneously; this is commonly referred to as macro diversity. Macro diversity has advantages such as providing improved coverage and capacity and improving user perceived quality. In the downlink (DL) the terminal can combine the transmission from all the cells involved, i.e. the active set, to get a better signal quality. The improved signal quality can be used to increase data rates, reduce delays and/or transmission power to obtain higher capacity. In the uplink (UL) all cells in the active set receive the signal from the mobile terminal and the transmission is successful if any of the cells can detect the signal. By adjusting the power to the momentarily best link the terminal can then use lower power and/or higher data rate. Macro diversity is more suitable for uplink transmission since no addition power or interference is needed.
The advantages of uplink macro diversity come with a cost of increased complexity and increased demand for hardware and transport resources in the network. All cells involved in the reception must reserve hardware resources for detection and decoding, also additional transport and processing recourses are needed. In current systems the set of cells that are included in the active set is based on radio propagation conditions, other resources are not considered. Furthermore, all cells that are included in the active set must participate in the reception.
In recent evolution of the uplink in CDMA systems such as the enhanced WCDMA uplink a fast scheduling functionality is introduced in the base station to improve resource management. Fast scheduling denotes the possibility for the base station to control when a mobile terminal is allowed transmit and at what rate. This enables fast reaction to the users momentary traffic demand and overload situations. Provided suitable estimates of the cell load, the system can be operated close to the maximum load limit providing improved user data rates as well as improved uplink capacity. In case of macro diversity, when two or more cells are involved in the reception, a serving cell is appointed with the main responsibility for scheduling the terminals transmission. The other cells in the active set may also influence the terminals transmission rate, but only in case the interference in the cell is too large. The non-serving cells may not reduce the terminal rate due to lack of other resources, such as hardware, processing or transport resources.
FIG. 1 shows a simplified illustration of the fast scheduling concept for enhanced uplink WCDMA. The mobile terminal request permission to transmit through a scheduling request. The scheduling request may for example consist of an “unhappy” indication from the happy bit included in the E-DPCCH (Enhanced Dedicated Channel (E-DCH) Dedicated Control Channel) or a scheduling information containing more details on the resource demands. The serving cell schedules the terminal and transmits a grant through the Enhanced Dedicated Channel (E-DCH) Absolute Grant Channel (E-AGCH). The grant allows the terminal to transmit non-scheduled data flows and sets an upper limit on the transport format, or rate, that the terminal may use. The grant can further be modified with the Relative Grant Channel (E-RGCH). For example, if the interference is too large, the non-serving cells in the active set may also influence the terminals rate through the E-RGCH.
The non-serving cells are not in advance aware of when and at what rate the serving cell will grant the terminal. In current solutions the non-serving cells must always be prepared and allocate hardware and other resources for the maximum possible rate. Due to limited resources the maximum rate allowed in Soft or Softer Handover (SHO) is then quite limited.
Alternatively the non-serving cell does not reserve or pre-allocate resources, but only allocates resources when needed, i.e. when the terminal starts to transmit. The problem with this solution is that it takes some time to allocate resources, specifically hardware, and during this time the non-serving cell will not be able to receive the signal. The serving cell may not necessarily have the best uplink signal quality due multi-path fading and/or uplink/downlink mismatch. In these cases the terminal transmission power will be adjusted to the best of the non-serving cells and there is a risk none of the cells will be able to receive the transmission. If none of the cells are able to detect the signal for some time this will give rise to a number of undesirable effects such as delay and impaired end-user quality as well as inefficient resource usage.